Russia, a main backer of President Bashar Assad, had offered a safe passage to opposition fighters who surrender in eastern Ghouta.

More than 1,000 people have been killed in the last two weeks of violence in eastern Ghouta, which borders the capital Damascus, according to Médecins sans Frontières, which has gathered data from hospitals in the area that it supports.

The Syrian army is making steady advances in the enclave, but it is facing a hostile West, which is threatening airstrikes to stop the push. The Syrian government, through the SANA state news agency, denied using chemical weapons.

State TV said 13 fighters and their families had arrived at the crossing point.

Keen to secure Damascus, government troops and allied militias have been closing in on the enclave since February 18 and have overrun more than half of it.

While ICRC officials expect this latest convoy to successfully deliver its cargo, humanitarian organisations say it is not almost enough aid for the estimated 400,000 people trapped in the area.

"After the opening of an additional checkpoint in the direction of Jisrein-Mlekha area, the Syrian side has registered increased number of clashes between the civilians and the militants who are blocking the exits from the area", Zolotukhin told reporters.

Hundreds of thousands of residents are still thought to live in the Eastern Ghouta, seven years into the civil war.

The roads connecting the towns to each other were all covered by army fire, the observatory said. Kurdish fighters insist the Turkish military has not yet besieged Afrin.

The YPG denies any direct organisational links to the PKK - an assertion backed by the U.S., which has provided the militia and allied Arab fighters with weapons and air support to help them battle the jihadist group Islamic State (IS).

UK: Ex Russian spy, daughter in 'serious condition'
Det Sgt Nick Bailey - who attended the scene on Sunday - is conscious but "very anxious" about being exposed to a nerve agent. According to local police chief Kier Pritchard, "around 21 people" have received treatment following the attack.